ST. LOUIS (AP) -- By the time the U.S. Supreme Court refused a last-minute request to stay the execution of Herbert Smulls, the Missouri inmate was already dead.

His attorneys said Thursday that it was the third straight case in which Missouri has moved ahead with an execution while the case was still in court.

While Missouri says it's perfectly legal to carry out executions before all appeals are exhausted, legal experts say the practice is rare. And the concerns from defense attorneys come as the death penalty in general is getting increased scrutiny amid separate questions about the drugs states are using to execute inmates.

Smulls, 56, was put to death late Wednesday after a protracted legal battle that lasted more than two decades, a legal fight that extended to his final day of life. Smulls was scheduled to die at 12:01 a.m. Wednesday, but various appeals pushed the case deep into the evening.

Attorneys for Smulls made one final push just before 10 p.m. for a stay from the U.S. Supreme Court, which had already ruled on other appeals that the execution could proceed. Timing was becoming a factor because the death warrant expired at 11:59 p.m. A new execution date would be required if Smulls was still alive at midnight.

He wasn't. The execution began at 10:11 p.m., and Smulls was pronounced dead at 10:20 p.m.

Joseph Luby, an attorney for Smulls, said he received an email at 10:30 p.m. from the Supreme Court, saying the stay application was denied at 10:24 p.m. - four minutes after Smulls was pronounced dead.

"It's just troubling and fundamentally lawless," Luby said.

Missouri Attorney General Chris Koster, in an emailed statement, said the state did nothing wrong.

"The United States Supreme Court has ruled that pending litigation is not sufficient to stop an execution," Koster wrote. "The legal mechanism for a federal court to stop an execution is a court-ordered stay."

Koster wrote that the state "directly asked the United States Supreme Court if the execution of Herbert Smulls should be stayed; for the third time that day, the Court said no. No stay of execution was in effect at the time of execution."

Missouri has executed three convicted killers in the past three months. Luby said that in all three cases, appeals were still pending.

One of the people executed was Allen Nicklasson. A three-judge panel of the 8th U.S. Circuit Court of Appeals had refused to grant a stay, but the full appeals court had not yet heard the case, Circuit Judge Kermit Bye wrote in a dissenting opinion days after that execution.

"In my near fourteen years on the bench, this is the first time I can recall this happening," the judge wrote.

Columbia Law School professor Jim Liebman said it is unusual, but not unheard of, for an execution to proceed with an appeal still pending. He agreed that corrections officials could reach the point of believing that enough is enough.

"You wouldn't be surprised if the state felt that way, maybe with justification, but the procedure is to have a court say that, not for the state to say that," Liebman said.

Texas, the nation's most active death penalty state, generally waits until all appeals are exhausted before carrying out executions.

In a few cases, the U.S. Supreme Court has put a halt to the stream of appeals by issuing an order that it would no longer accept petitions in a case, Liebman said. The court issued no such halt in Smulls' case.

"What that does is sort of put a legal end to the matter," Liebman said.

Attorneys for Smulls filed several appeals in the weeks and days leading up to the execution, and on the final day.

Many centered on concerns about the way Missouri obtains its execution drug from an unnamed compounding pharmacy. At least two recent executions have raised concerns about the effectiveness of drugs used in executions, putting increased scrutiny on how states carry out the death penalty. Two weeks ago, Ohio inmate Dennis McGuire took 26 minutes to die by injection, gasping repeatedly as he lay on a gurney with his mouth opening and closing. And on Jan. 9, Oklahoma inmate Michael Lee Wilson's final words were, "I feel my whole body burning."

Some of the other appeals centered on that fact that Smulls, who was black, was convicted by an all-white jury.

Florence Honickman called all the appeals a "travesty." She has permanent injuries after being shot by Smulls in the 1991 jewelry store robbery in suburban St. Louis that killed her husband, Stephen.

Honickman and her two adult children spent most of Tuesday and Wednesday at the prison in Bonne Terre, Mo., waiting through the late appeals until finally witnessing Smulls' death.

"I believe it is cruel and unacceptable both to the criminal about to be executed and to the victims' family that after endless appeals, 20-plus years on death row and after the execution date has been set, the criminal is given hope of a stay based on absurd technical and procedural matters," Florence Honickman said.